Transcript Light & Eye

Light & Eye
K.7 SOL
The student will investigate and
understand that shadows occur
when light is blocked by an
object. Key concepts include
*shadows occur in nature when
sunlight is blocked by an object;
and * shadows can be produced
by blocking artificial light
sources.
5.3 SOL
The student will investigate and understand
basic characteristics of white light. Key
concepts include * the visible spectrum,
light waves, reflection, refraction, diffraction,
opaque, transparent, translucent; *optical
tools (eyeglasses), lenses, flashlight,
camera, kaleidoscope, binoculars,
microscope, light boxes, telescope, prism,
spectroscope, mirrors); and * historical
contributions in understanding light.
What is Light?
The nature of light
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Light is a form of energy
Light interacts differentially
with matter
Vision occurs as light
energy is processed by the
human eye
Light is Energy
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Light is a form of radiant
energy. It travels in waves.
Visible light is part of the
electromagnetic spectrum
Radiant energy travels in
straight lines
Wave characteristics
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Wave length: Distance
between Maximas
Frequency: The number of
waves per second
Energy
Relationships
Wavelength = 

Frequency = v (hz) (cps)
1, 2, 3 have passed me
Light is fast
Frequency and Velocity
c = velocity of light in a vacuum
 c is constant
 c = 300 million m/sec
   c = /sec x m/
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The Electromagnetic
Spectrum
A continuous spectrum of
electromagnetic waves
(Maxwell, 1880),
(Hertz,1888) with varying
wavelengths, frequencies
and energies.
The Electromagnetic Spectrum
Energy
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E = mc2
Energy = mass x velocity
squared
As velocity increases,
energy increases
Relationships
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As wavelength,  increases
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frequency, , decreases
energy
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decreases.
As wavelength,  decreases
frequency,
energy
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increases
Visible light
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A portion of the EM spectrum
  700 nm - 400 nm
ROYGBIV
Infrared - longer than red
Ultraviolet - shorter than violet
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Which has the most energy?
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T coor  eergy
Color
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A wonderful resource http://www.thetech.org/exhi
bits_events/online/color/ov
erview/
White light & Prisms & Newton
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Newton conducted prism experiments
White light (sun light) contains all the
visible wave lengths of light
Rainbow
When white
light is
broken
(refracted)
by rain
drops into
the visible
spectrum
White
Rattlesnake- Infrared Sensor
Fluorescence
Light is absorbed at one wavelength
(energy) (color) and emitted at
another
Light interacts with
matter
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reflection
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refraction
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diffraction
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absorption/transmission
Interactions depend on
the nature of matter and
light
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The denser the matter the
more interactions
The greater the energy of
light the more interactions.
Why is the sky blue and
the sun yellow?
Reflection
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light returns after striking
an object
the angle of reflection
equals the angle of
incidence
Angles of Reflection
Normal, at 90o from surface
Angles of reflection
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Angle of incidence = Angle
of reflection
Normal is an imaginary line
perpendicular to the
reflecting surface
Activity: Reflection
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Bouncing light off mirrors
 using
a flashlight and mirror in a dark room.
Place a mirror on the floor and shine the light
on the mirror at an angle. Hold a book in the
path of the reflected light. Using a protractor
can you measure the angle? (Ticotsky, p 30)
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The Case of the Vanishing Reflection
 Carefully
cut 8 - 10 inches of foil and look at
your reflection. Crinkle the foil and flatten it
again. Can you still see your reflection?
(Churchill, p 95)
Refraction
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Light is bent when it traveled from one
medium to another of a different density.
Snell(1621) The law of refraction
 When
the light passes through a denser
medium, the light is bent toward the normal,
because the light slows down (velocity
decreases).
 When light passes through a less dense
medium the light is bent away from the
normal.
AIR
WATER
GLASS
AIR
Refraction occurs at the interface between
two mediums
Water more dense than air-->
Air less dense than glass -->
Activity: Refraction
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You can see this effect when
you place a coin under a clear
glass. Look straight down, what
do you see? Now look from an
angle. What happens?
Turn this around. Place a coin in
a bowl and move so that you
can not see it. Have your
partner slowly add water to the
bowl, what happens
Diffraction
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light that passes very close
to an edge bends
light
Obstacle
Diffracted
Theoretical
Transmission Absorption
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Light that passes through
an object is transmitted
Light neither transmitted
nor reflected is absorbed
Transmitted light strikes
the retina of the eye to
stimulate vision
Transparent
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All light is transmitted
Opaque
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No transmission. Only reflects
and absorbs.
Translucent
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Reflects, transmits and
absorbs
Absorbed light causes
the heating effect
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Prove that dark colors absorb light and
light colors reflect light.
 Put
two cups containing water and a
thermometer behind a white sheet of
paper and a black sheet of paper. Place
a lamp very close to the cups, allow the
lamp to shine only on the paper not the
thermometer or water. What happens?
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What colors do we wear in the summer?
Why?
Light travels in a straight line
What Path Does Light Follow?
Set
up 3 or 4 index cards standing up.
Punch a hole in each card. With a
flashlight on one side, move the cards
until the light passes through each.
Look through the last back at the light.
What will you see? Move the cards,
what happens?
Shadows
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Shadows are a consequence of light moving
in a straight line.
If an object passes in front of the light source,
the light is absorbed(blocked) and a shadow
forms.
Shadows are always darkest in the middle and
lighter on the edges.
Why Do Shadows
Change in Size?
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The more light rays that are
blocked by the object, the
larger the shadow.
The closer the object to the
light source the BIGGER
the shadow
Activity: Shadow
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Place a projector across
the room from a black wall,
turn the projector on and
the lights off. Slowly walk
toward the projector. What
happens to the size of the
shadow formed?
Why are shadows darker
in the middle, lighter at
the edges?
Diffraction
Optical tools
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Lenses
Eyeglasses
Microscope
Camera
Kaleidoscope
Binoculars
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Flashlight
Light boxes
Telescope
Prism
Spectroscope
Mirror
Lens
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Lenses alter the angle of incidence, and
thus the angle of refraction of light rays
Lenses determine how much light rays
are bent
Lenses determine in which direction light
rays are bent
Lenses bring light rays together
(converge)
Lenses spread light rays apart (diverge)
Magnifying lens
Tools that use lenses /
mirrors
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Eyeglasses - to correct poor
vision
Telescopes- to see things that
are far away
Microscope - to see very small
things
Binoculars - to enlarge an image
Camera - focus image on film
Binoculars
Telescope
Telescope
Mirror
Kaleidoscope
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Sir David Brewster,
1817
internal mirrors
repeat the pattern
http://kaleidoscope
heaven.org/info.ht
ml
Lasers
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Light Amplification by the
Stimulation Emission of Radiation
History
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Mesopotamia
Ptolemy
Classical Greeks
Pythagoreans
Newton
Young & Helmholtz
Mesopotamia (1500 BC)
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polished metal used as mirror
piece of glass used as burning
lens
Ptolemy (127-135 AD)
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Alexandria
rules of additive light mixtures
spinning disks*
light bends when passing
through glass (refraction)
Wrote Optica on optical
phenomena
Classical Greeks
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Sought underlying principles
Observation without
experimentation
Light travels in straight lines
Reflected light: the angle of
incidence = the angle of reflection
Plato:
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believed that light was
emitted by the eye.
This was believed as late
as 1644 when Descartes
published a book
elaborating a similar
theory.
Aristotle (350 BC)
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all colors are derived from mixture of black
and white, White light is the purest color,
others are contaminated
“These juxtapositions involving simple ratios
may the most pleasing colors such as purple
or crimson, like the concords in music.
Irrational ratios may produce impure colors”
Sense and the Sensible
Isaac Newton (1600’s)
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Beginning of modern scientific
understanding /testing
Prisms
white light consists of light of
may different colors with
different “refrangibility” and
power
Perception of color is due to
physical properties of light
Make a prism
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Make a prism by placing a
small mirror in about an
inch of water in a baking
pan. Lean the mirror
against one edge near
bright sun light and direct
the reflection to a white
surface.
Young (1802) & Helmholtz
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trichromatic (three color receptor)
theory of color vision
demonstrated that the wide range of
colors can be reproduced by
superimposing various proportions of
red, green and blue light
there are three color receptors in the
eye
Today
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the three cone pigments
genes have been located
on specific chromosomes
Human Eye
Human eye structures
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Cornea - most anterior
Aqueous humor - watery
solution behind cornea
Lens - fairly rigid structure
Vitreous Humor- jelly like colloid
Retina -delicate light receiving
layer
Sclera - tough protective white
outer layer
Aqueous humor
cornea
Vitreous
Humor
Refractive surfaces
Medium
Refractive index
Air
1
Cornea
1.3
Aqueous humor 1.3
Lens
1.42
Vitreous
1.42
Where does most
refraction occur?
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At the air/cornea interface.
This is the site of astigmatism
A normal cornea is like a
baseball - evenly curved in all
directions
An astigmatic cornea is like a
football, more curved in one
direction than the other.
Aqueous humor
cornea
Vitreous Humor
Human Lens
The
lens in the human eye is
convex, but unlike a glass lens, it is
elastic so that it can change shape
to focus on objects at varying
distances. The lens becomes short
and fat when viewing close objects
and elongated and thin when
viewing distant objects.
Hyperopia (far sighted)
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Sometimes eye muscles are unable to
focus light on the retina, the screen at the
back of the eyeball. If the image forms
behind the retina for nearby objects,
farsightedness (results. Convex lenses are
prescribed for hyperopics to assist the eye
in making light converge on the retina for
nearby objects
Hyperopia & Myopia
Presbyopia - old eyes
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As the eye ages, the lens
becomes less transparent and
absorbs more blue light.
What is the effect on vision?
Cataracts
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A loss of transparency usually of the cornea
May be congenital or
aquired (uv light)
What is the effect on vision?
Vision occurs as
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Light energy collected by
the eye
Is transduced in the retina
to neural energy
Travels via nerves
and is processed by the
brain
Color vision
The eye has
three types
of light
receiving
units, red,
green and
blue cones
Differential stimulation
of cones
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If you stimulate
only red and
green cones,
not blue, you
see YELLOW
Try this with
different colors
of cellophane on
an overhead
projector or tied
over flash lights.
White
Colorblindness
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the British
Navy
The result
of cone
(pigment)
dysfunction